This application proposes to use the Siamese fighting fish as a model system for studying the neural mechanisms underlying aggression. This fish is one of the most aggressive animals known. It engages in agonistic display and fights with conspecifics under almost any circumstances. The agonistic display is characterized by a repertoire of stereotyped postures: including tonic gill cover extension, fin erection, and color changes. Since this display never occurs spontaneously, and inevitably precedes actually fighting, it is a reliable indication of the aggressive state of the animal. Gill cover extension in the fish is mediated by a single muscle, the dilator operculi (DO), which controls both the rhythmic respiratory movement and the tonic agonistic display of the operculum. Tonic firing of DO motoneurons is thus a physiological manifestation of the aggressive state, and will be used in this study as an entry into understanding the central control of aggression. Four sets of experiments are proposed: (1) Extracellular recording will be used to define physiological properties of DO motoneurons. A display will be elicited under experimental conditions to examine the regulation of these motoneurons during a switch from a rhythmic to a tonic firing mode. This will provide a cellular measure of the aggressive state in the CNS. (2) Neuro-anatomical tracing will be done to identify brain regions that provide inputs to the motor nuclei. This helps to piece together the circuitry that regulates the aggressive state. (3) Intracellular recordings will be attempted to study the nature of synaptic inputs that control the firing patterns of DO motoneurons. (4) The influences of sex as well as some experiential factors on the aggressive display will be examined. Animals with enhanced or suppressed aggressiveness will be produced. Their aggressive states will be correlated with any changes in the physiological properties of the DO motoneurons and brain regions that regulate motoneuron activities. These results will help to pinpoint where and how hormonal and experiential factors act in the CNS to influence aggressive behavior. Violence and aggression are urgent problems in our society. While much studies on its social and behavioral aspects have been made, little is known about the neural mechanisms underlying aggression, partly because animal models amenable to detailed cellular physiological studies are rare. The Siamese fighting fish may be a valuable model system for understanding the cellular bases of aggression. Only when we understand these bases will we be able to intervene with the aggressive process. The proposed studies aim at searching for fundamental and general rules that may be useful in indicating the directions that aggression research in higher mammals, including humans, should take, the kind of variables that should be considered, and hypotheses that should be tested.